Because of the hysteresis behavior of a viscoslastic rubber material, a tread portion of a tire repeats deformation and contraction during the rotational motion of the tire and thereby heats up. When the rubber material constituting the tread portion increases in amount, the hysteresis loss due to bending deformation and shear deformation caused by the rotational motion of the tire increases, as well. For this reason, a tire having a thick tread portion is likely to have a high temperature.
Especially, a heavy duty tire for a large vehicle used at a mine or a construction site is characterized by its tendency to heat up owing not only to the large amount of rubber material used, but also to the repeated deformation and contraction of the tire used in a heavy load state, on a poor road surface, and under harsh traction conditions. When a tire has a high temperature when rotating, troubles such as separation between the rubber material which forms the tread portion and a belt layer may occur. This contributes to shortening of the tire replacement cycle.
In this respect, the following method is conventionally known, specifically, secondary grooves extending in a tread width direction are formed in the tread portion in order to reduce the amount of rubber material, which is a factor of the heat generation, and also to increase the surface area of the tread portion so that heat dissipation is promoted in the tread portion (for example, Patent Document 1).
However, such a conventional tire has the following problem. Specifically, although heat dissipation can be promoted by forming lateral groove portions (secondary grooves) intersecting with the tire circumferential direction so as to increase the groove area, the increase in the groove area leads to decrease in the rigidity and wear resistance of the tread portion. In this way, the dissipation performance of a tire and the rigidity of a tire have a trade-off relation. Accordingly, there is a limit on obtaining a high dissipation performance by increasing the groove area.